Dental caries is a serious public health issue throughout the U.S., especially more evident in underserved and elderly populations. When preventative measures fail, the best case scenario is for restorations to be long lasting and reliable. Unfortunately, even with the major advances of adhesive dentistry, hundreds of thousands of resin composite restorations have to be replaced each year. The average life-span of this type of restoration is 5-10 years or lower, depending on the caries risk level of the patient (Demarco F F Dental Materials 28, 87-101 (2012); incorporated by reference herein). The causes of premature failure are complex; however, contributing factors include polymerization stress (Park H Y et al Dental Materials 28, 888-893 (2012)); incorporated by reference herein)—which immediately challenges the bonded interface—and fracture (Demarco 2012 supra). The vast majority of current commercially available resin systems are based on dimethacrylate polymerizations that undergo vitrification at relatively early stages of conversion. Early vitrification increases the potential for stress development at both the bonded interface and in the bulk of the material.
In spite of the many efforts to improve dental resin composite formulations (Stansbury J W et al, J Dent Res 71, 1408-1412 (1992); Carioscia J A et al, Dent Mater 21, 1137-1143 (2005); Weinmann W et al, Dent Mater 21, 68-74 (2005); Trujillo-Lemon M, J Polymer Sci Part A—Polymer Chem 44, 3921-3929 (2006); all of which are incorporated by reference herein) replacement of restorations failing from secondary decay and fracture is a common occurrence in daily practice, costing millions of dollars annually. Restoration failure has been attributed to (i) degradation of the tooth-restoration interface and polymerization stress, resulting in gap formation (Carvalho R M et al, Dental Materials 28, 72-86 (2012); incorporated by reference herein) and (ii) residual stress concentrating within the bulk of the material (Park J W, Dental Materials 21, 882-889 (2005); incorporated by reference herein).
To date, materials developed to reduce shrinkage and stress have proven inefficient at extending the service life of restorations (Burke F J T et al, Dental Materials 27, 622-630 (2011); incorporated by reference herein). Furthermore, clinical research has failed to show a correlation between gap formation and secondary decay. This stems from the multi-factorial nature of dental caries, as well as the extremely technique-sensitive placement of composite restorations by practitioners. In addition, stress reductions reported for the few “low-shrink” materials available ranged from 5 to 25% (Boaro L C C, Dental Materials 26, 1144-1159 (2010) and Li Y et al, J Appl Polymer Sci 124, 436-443 (2012); both of which are incorporated by reference herein). This level of stress reduction may not be sufficient to produce appreciable clinical improvement. Clearly the problem of shrinkage and stress still persists, and millions of restorations are replaced every year due to fracture and marginal failure.